1. Field of the Invention
The present invention relates generally to an optical fiber. More particularly, the present invention relates to a double-coated optical fiber and its coating strip force.
2. Description of the Related Art
In general, an optical fiber comprises a core that provides a light transmitting medium, a cladding that surrounds the core for trapping light in the core; A coating layer surrounds the cladding, for protecting the inside of the fiber from an external environment.
The coating layer is a rather significant component that determines physical fiber optic characteristics such as bending, chemical resistance, and mechanical strength. In order to measure the mechanical characteristics of an optical fiber, it is common to the art that both a dynamic fatigue test and a coating strip test are performed to evaluate the quality the optical fiber.
The dynamic fatigue test is used to estimate the mechanical life and the degradation of the optical fiber during usage, because the optical fiber undergoes physical changes due to external factors when it is applied in actual field use.
A dynamic stress corrosion parameter (Nd) is defined as a ratio of the change of fracture stress distribution with respect to the change of an applied stress rate. The dynamic stress corrosion parameter is equal to or greater than 18 in accordance with FOTP-28 under the test standard of GR-20-CORE 4.4.4. The coating strip test is normally performed using a tensile testing machine that measures the tensile force to strip the coating from the surface of the cladding. The tensile force is also referred to as a coating strip force. The coating strip force testing tests for the adhesive force of the coating layer.
The peak value shall range from 1.0 to 9.0N in accordance with FOTP-178 under GR-20-CORE 4.4.2.
If the coating strip force is too great, this value is not ideal in that it leads to fiber breakage and excessive coating residue. However, on the contrary, if the coating strip force is too small, the adhesive force between the optical fiber and the coating layer is insufficient and coating delamination occurs. Therefore, it is of extreme importance to set the coating strip force appropriately.
With regard to the mechanical characteristics of a double-coated optical fiber, UV curing conditions and physical, chemical, and thermal tendencies are well known. However, with regard to the characteristics over a passage of time, the geometrical coating structure and mechanical characteristics of the optical fiber is unknown. For example, there is a specification that the diameter of an optical fiber is 125±1.0 μm and the outer diameter of a secondary coating layer is 245±10 μm under the existing provisions. Yet, there is no explicit specification for the outer diameter of a primary coating layer (test standards: IEC 60793-2, GR-20-CORE, TIA/EIA-492c000, etc.).